This invention relates to improved membranes for the separation of fluids from blends of specific polyimides and polyamides. Membranes fabricated from these blends exhibit a particularly useful combination of fluid-separating properties, especially for the separation of carbon dioxide from hydrocarbons and exhibit improved mechanical strength compared to the polyimide component alone.
Permselective membranes for fluid separation are known and used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane, or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical, and oil refining streams. For certain fluid streams, one or more component or minor contaminant may exhibit a strong interaction with the material of the membrane, which can plasticize the membrane. This can result in reduced productivity and selectivity, and ultimately, loss in membrane performance. Furthermore, some membrane materials may offer resistance to the interaction with contaminants, but suffer from poor mechanical properties, resulting in membrane failure when exposed to high membrane differential pressures and high temperatures. Other materials may not be capable of processing into membranes of the desired configuration, such as a hollow fiber membrane. A membrane with a good balance of high productivity and selectivity for the fluids of interest, and persistently good separation performance, despite long-term contact with aggressive process composition, pressure and temperature conditions, and that can be processed into a wide variety of membrane configurations is highly desired.
Polymeric blending has traditionally been thought to be either problematic or of no benefit in the membrane field, primarily because different polymers are generally not miscible with one another, and for those few polymers that are miscible, offer no blending advantage because of various reasons, including difficulty in blending, poor mechanical properties, and limited range of fluid transport properties.
The references discussed below describe separation membranes known in the art and disclose information relevant to production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical and oil refining streams. However, these references suffer from one or more of the disadvantages discussed above.
U.S. Pat. No. 4,705,540 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines, having substituents on all positions ortho to the amine functions, and a rigid dianhydride or mixtures thereof, specifically pyromellitic dianhydride (PMDA), and 4,4′-(hexafluoroisopropylidene)-bis (phthalic anhydride) (6FDA). These polyimides form membranes with high gas permeabilities, but fairly low permselectivities. These polyimides are also sensitive to various organic solvents.
U.S. Pat. No. 4,717,393 shows that polyimides incorporating, at least, in part 3,3′,4,4′-benzophenone tetracarboxylic dianhydride, and phenylene diamines, having substituents on all positions ortho to the amine functions can be photo chemically crosslinked. Membranes formed from this class of crosslinked polyimides have improved environmental stability and superior gas selectivity than the corresponding crosslinked polyimide. However, photochemical crosslinking is not truly a practical method for fabricating cost-effective gas separation membranes.
U.S. Pat. No. 4,880,442 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines, having substituents on all positions ortho to the amine functions, and essentially non-rigid dianhydrides. These polyimides again exhibit high gas permeabilities, but once again low permselectivities.
Bos, et. al., AlChE Journal, 47,1088 (2001), reports that polymer blends of Matrimid® 5218 polyimide (3,3′,4,4′-benzophenone tetracarboxylic dianhydride and diaminophenylindane), and copolyimide P84 [copolyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride, and 80% toluenediisocyanate/20% 4,4′-methylene-bis(phenylisocyanate)] can increase the stability of the membrane against carbon dioxide plasticization when compared to the plain Matrimid® 5218 membrane. Other polyimide blends or blends with polyamides or polyamide-imide for use as gas separation are not disclosed.
U. S. Pat. No. 5,055,116 describes a blend of aromatic polyimides, in which the proportion of the polymer components is adjusted to achieve certain permeability and selectivity of a polymer membrane. The final properties of a new polymer membrane may be predicted so that a membrane with those desired final properties could then be manufactured. U.S. Pat. No. 5,055,116, also indicates that the gas transport properties of the membrane prepared from the polyimide blends are predictable and the membrane may be “engineered” to achieve the desired final properties. To the contrary, the gas transport properties of the present invention are unpredictable and surprisingly good.
U.S. Pat. No. 5,635,067 discloses a fluid separation membrane based on a blend of two distinct polyimides. One is the copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA), and optionally pyromellitic dianhydride (PMDA) with a mixture of toluene diisocyanate, and/or 4,4′-methylene-bis(phenylisocyanate). The other is Matrimid® 5218 polyimide.
Barsema, et al., (Journal of Membrane Science, 216 (2003), p 195-205, reports the permeation performance of dense film and asymmetric hollow fiber membranes made from the copolymer derived from reacting benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) with a mixture of toluenediisocyanate, and/or 4,4′-methylene-bis(phenylisocyanate).
U.S. Pat. Nos. 4,532,041, 4,571,444, 4,606,903, 4,836,927, 5,133,867, 6,180,008, and 6,187,987, disclose membranes based on a polyimide copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA), and a mixture of di(4-aminophenyl)methane, and a mixture of toluene diamines useful for liquid separations.
U.S. Pat. Nos. 5,605,627, 5,683,584, and 5,762,798, disclose asymmetric, microporous membranes based on a polyimide copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA), and a mixture of di (4-aminophenyl)methane, and a mixture of toluene diamines useful for liquid filtration or dialysis membranes.
Accordingly, it is highly desirable to create a membrane that can be used commercially in applications, such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting, and blanketing, separation of carbon dioxide from methane, or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical and oil refining streams. The desired membranes should exhibit a resistance to interaction of the material with the process and the resulting plasticizing of the membrane. Furthermore, membranes should have superior mechanical properties to allow the use of the membranes in high differential pressure applications, and should be capable of processing into membranes of the desired configuration (such as hollow fiber membranes). Thus, membranes with a good balance of high productivity and selectivity for the fluids of interest, and persistently good separation performance despite long-term contact with aggressive process composition, pressure and temperature conditions are desired.
As used in this application, a “repeating unit” in a polymer is a molecular segment in the polymer chain backbone that repeats itself regularly along the polymer chain. In this respect, the term repeating units is meant to cover all portions of such polymers, and any number of the repeating units.
As used in this application, a “mixed polymer”, is a molecularly miscible blend of at least two polymers.